PHYSIOLOGICAL PROCESSES DURING FLOWERING 447 



of time before they attain the abiUty to germinate. As is 

 known, they must be subjected to stratification, which consists 

 in their being kept for several weeks in a moist state at a tem- 

 perature sHghtly above 0°C. In other plants, as for instance in 

 watermelons, the flesh of the fruit contains special substances 

 of the type of hormones that inhibit germination, which becomes 

 possible only after the release of the seeds from the flesh. This 

 occurs under natural conditions when animals devour the fruits. 

 Germination is also inhibited by the comparatively high cell-sap 

 concentration of the flesh. 



When fruits are overripe, these inhibit! ve factors are removed, 

 and the seeds become capable of germinating. But sometimes 

 such inhibitive factors for some unknown cause are absent in 

 succulent fruits, and then mass germination of seeds occurs 

 inside the fruit that has not yet completely ripened, as is the 

 case with the citrus and many tropical fruits. 



102. Influence of Environmental Conditions in the Course of 

 Ripening. Shriveled Grain and Its Causes. Dependence of 

 the Chemical Composition of Fruit and Seeds on Geographical 

 Factors. — The successful filling and ripening of grain depend 

 to a great degree upon the environmental conditions during this 

 period. Cases are known when prospects of a good peld were 

 entirely shattered in consequence of unfavorable weather 

 conditions. 



High temperatures and dryness of the air with high winds 

 bring great disasters to agriculture in the southeastern and 

 eastern provinces of Russia and in the Great Plains of the United 

 States, owing to poor filling of grain. Two or three days or 

 sometimes even a few hours of such dry winds may induce 

 premature ripening of the grain, which- remains insufficiently 

 filled and shrivels, the yield thereby being decreased to one-half 

 or one-third. The main cause of tfiis phenomenon is the dis- 

 turbance of the normal flow of nutrient substances from the 

 leaves to the ripening seeds. A sufficient turgidity of all the 

 organs of the plant, especially of the leaves, which are the source 

 of carbohydrates, is necessary for producing this normal flow. 

 According to Miinch's scheme, examined in Art. 79, the prune 

 mover of the current of elaborated substances is the turgor 

 pressure of the cells that form the assimilates; and when this 

 tension falls during dry winds, the translocation of assimilates 



